linux-kernel - Re: [PATCH tip/core/rcu 09/12] fs: Substitute rcu_access_pointer() for rcu_dereference

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Date:	Mon, 17 Feb 2014 14:00:15 -0800
From:	Josh Triplett <josh@...htriplett.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...nel.org,
	laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	niv@...ibm.com, tglx@...utronix.de, peterz@...radead.org,
	rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
	darren@...art.com, fweisbec@...il.com, oleg@...hat.com,
	sbw@....edu, Alexander Viro <viro@...iv.linux.org.uk>,
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH tip/core/rcu 09/12] fs: Substitute rcu_access_pointer()
 for rcu_dereference_raw()

On Mon, Feb 17, 2014 at 01:35:56PM -0800, Paul E. McKenney wrote:
> From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
> 
> (Trivial patch.)
> 
> If the code is looking at the RCU-protected pointer itself, but not
> dereferencing it, the rcu_dereference() functions can be downgraded to
> rcu_access_pointer().  This commit makes this downgrade in __alloc_fd(),
> which simply compares the RCU-protected pointer against NULL with no
> dereferencing.
> 
> Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> Cc: Alexander Viro <viro@...iv.linux.org.uk>
> Cc: linux-fsdevel@...r.kernel.org

I'm beginning to wonder if this common pattern ought to have an
rcu_pointer_is_null(), which would not return the pointer, only the
boolean.

Regardless, for this patch:
Reviewed-by: Josh Triplett <josh@...htriplett.org>

>  fs/file.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/file.c b/fs/file.c
> index db25c2bdfe46..18f7d27855c4 100644
> --- a/fs/file.c
> +++ b/fs/file.c
> @@ -497,7 +497,7 @@ repeat:
>  	error = fd;
>  #if 1
>  	/* Sanity check */
> -	if (rcu_dereference_raw(fdt->fd[fd]) != NULL) {
> +	if (rcu_access_pointer(fdt->fd[fd]) != NULL) {
>  		printk(KERN_WARNING "alloc_fd: slot %d not NULL!\n", fd);
>  		rcu_assign_pointer(fdt->fd[fd], NULL);
>  	}
> -- 
> 1.8.1.5
> 
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